1. Field of the Invention
The present invention relates to a pressure sensor having a metal diaphragm responsive to pressure to be measured.
2. Description of Related Art
An example of a pressure sensor having a metal diaphragm responsive to pressure is disclosed in JP-A-7-243926. The essence of the pressure sensor is illustrated in FIG. 5 attached hereto. This pressure sensor is a diaphragm-seal type pressure sensor having a connector case J1 and a housing J2 connected to each other for forming a pressure-detecting chamber J5 therebetween. A bottom end of the connector case J1 is inserted into a mounting space J4 of the housing J2, and an upper end of a wall forming the mounting space J4 is staked against a shoulder of the connecting case J1. A ring-weld J9, to which an outer peripheral portion of a metal diaphragm J3 is connected, is sandwiched between the connector case J1 and the housing J2.
A sensing element J6 is disposed in the pressure-detecting chamber J5 enclosed by the metal diaphragm J3. The pressure-detecting chamber J5 is filled with encapsulated liquid J8 for transferring pressure imposed on the metal diaphragm J3 to the sensing element J6. A portion connecting the connector case J1 and the housing J2 is sealed with an O-ring J7. The pressure sensor is connected to a pipe or a block containing fluid therein by a screw formed at a bottom portion of the housing J2. A pressure introduced into the pressure sensor is imposed on the metal diaphragm J3 and detected by the sensing element J6.
To prevent fluid introduced into the pressure sensor from leaking through a portion connecting the ring-weld J9 and the housing J2, the ring-weld J9 and the housing J2 are made of a same material and welded together at a contacting portion J10. In order to obtain a firm welding strength between the ring-weld J9 and the housing J2, both have to be made of the same material, such as carbon steel S15C or SUS.
On the other hand, the pipe or the block to which the pressure sensor is connected is often made of a material such as aluminum. If the housing J2 made of carbon steel is connected to a pipe made of aluminum, a potential difference is generated between two different materials contacting each other. Such a potential difference may be as high as 0.5 volts. It is highly probable that corrosion by battery-effect will occur in the pipe made of aluminum. To avoid such corrosion, it is conceivable to make the housing J2 with the same material, such as aluminum, as the pipe to which the pressure sensor is connected. However, it becomes considerably difficult to weld the ring-weld J9 made of SUS or the like to the housing J2 made of aluminum. This is because the melting point of SUS is considerably higher than that of aluminum. Further, it is very difficult to change the material of the metal diaphragm J3 and the ring-weld J9 to other materials because the material such as SUS or the like is most suitable for these parts to obtain high reliability and strength of the welded portion.